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null (C# Reference)

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The null keyword is a literal that represents a null reference, one that does not refer to any object. null is the default value of reference-type variables. Ordinary value types cannot be null. However, C# 2.0 introduced nullable value types. See Nullable Types.

The following example demonstrates some behaviors of the null keyword:

class Program
{
class MyClass
{
public void MyMethod() { }
}
static void Main(string[] args)
{
// Set a breakpoint here to see that mc = null.
// However, the compiler considers it "unassigned."
// and generates a compiler error if you try to
// use the variable.
MyClass mc;
// Now the variable can be used, but...
mc = null;
// ... a method call on a null object raises
// a run-time NullReferenceException.
// Uncomment the following line to see for yourself.
// mc.MyMethod();
// Now mc has a value.
mc = new MyClass();
// You can call its method.
mc.MyMethod();
// Set mc to null again. The object it referenced
// is no longer accessible and can now be garbage-collected.
mc = null;
// A null string is not the same as an empty string.
string s = null;
string t = String.Empty; // Logically the same as ""
// Equals applied to any null object returns false.
bool b = (t.Equals(s));
Console.WriteLine(b);
// Equality operator also returns false when one
// operand is null.
Console.WriteLine("Empty string {0} null string", s == t ? "equals": "does not equal");
// Returns true.
Console.WriteLine("null == null is {0}", null == null);
// A value type cannot be null
// int i = null; // Compiler error!
// Use a nullable value type instead:
int? i = null;
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}

C# language specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.